SUNRPC: fix variable type

Due to incorrect len type bc_send_request returned always zero.

The problem has been detected using proposed semantic patch
scripts/coccinelle/tests/assign_signed_to_unsigned.cocci [1].

[1]: http://permalink.gmane.org/gmane.linux.kernel/2046107



Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>